Copper based precipitation hardening alloy

ABSTRACT

The invention relates to a copper based precipitation hardenable alloy containing at least one of the elements chromium, zirconium or titanium, wherein the copper based precipitation hardenable alloy is alloyed by phosphorus.

This invention relates to at least one copper based precipitation hardenable alloy in which high strength mechanical properties and an anneal resistance are improved.

The JP patent publication 06-212,374 describes a method for manufacturing a copper based precipitation hardenable alloy to be used as material of small electric and electronic components. The alloy contains by weight 2 to 4% nickel, 0.5 to 1.0% silicon, 0.1 to 1.0% zinc, 0.001 to 0.15% aluminium, 0.01 to 0.1% manganese and 0.001 to 0.1% chromium. Because of many alloying elements the manufacture of the alloy is more expensive than an alloy having less alloying elements.

The copper based precipitation hardenable alloy in the GB patent 609,900 contains by weight 0.25 to 1.5% chromium with deoxidants of zinc, boron, sodium, lithium and phosphorus in amounts not exceeding 0.2% as well as strengthening elements of nickel, iron or cobalt in amounts between 0.1 and 5.0%. In this alloy of the GB patent 609,900 the value for the electrical conductivity is between 69 and 74% IASC (International Annealed Copper Standard).

The electrical conductivity and strength of copper is dependent upon the purity of copper. Moreover high purity copper is too soft for many applications where high strength mechanical properties and an anneal resistance are required. Direct alloying of copper has considerable disadvantages since the direct alloying has an inverse relationship on the conductivity of copper. A beneficial way of producing a high strength copper alloy with good electrical properties is to select an alloying element that forms a precipitate within the copper. The advantage with precipitation hardenable copper alloys is the amount of alloying required which is low and once aged the electrical conductivities greater than 85% IASC can be achieved. However, the requirements in the properties of the precipitation hardenable copper based alloys are today increased especially in the electrical conductivity with new solutions for instance in electric, electronic or welding industry where these alloys are planned to be used.

The object of the present invention is to eliminate some drawbacks of the prior art and to achieve improved precipitation hardenable copper alloys, which include improvements in the properties of the alloys having at least a decreased resistance to sticking, and increased conductivity when comparing with the prior art. The essential features of the present invention are enlisted in the appended claims.

In accordance with the invention copper-chromium (CuCr), copper-chromium-zirconium (CuCrZr), copper-zirconium (CuZr) or copper-titanium (CuTi) precipitation hardenable alloys contain phosphorus as an alloying element in the range of 100 to 500 parts per million (ppm). The addition of phosphorus to the chromium, zirconium or titanium bearing copper has a significant effect on the hardness and on electrical properties.

The copper-chromium (CuCr), copper-chromium-zirconium (CuCrZr), copper-zirconium (CuZr) or copper titanium (CuTi) precipitation hardenable alloys in accordance with the invention contain 0.1-1.5% by weight chromium and/or 0.01-0.25% by weight zirconium or 0.05-3.4% by weight titanium, the remainder being copper and the usual impurities. The copper content in the alloys containing chromium and/or zirconium is at least 98.5% by weight copper and in the alloy containing titanium at least 96.5% by weight copper.

The alloying element, phosphorus, in the copper alloys forms phosphides and this formation of phosphides has been found an impact on the electrical conductivity and mechanical strength for the precipitation hardenable copper alloys. When phosphorus is added as an alloying element to these copper chromium (CuCr), copper chromium zirconium (CuCrZr), copper zirconium (CuZr) or copper titanium (CuTi) precipitation hardenable alloys in accordance with the invention, phosphides can form during heat treatment or even during casting. It has been discovered with the invention that phosphorus additions of up to 550 parts per million (ppm) have an advantageous effect on the electrical and mechanical properties of the alloys. The formation of phosphides causes coarsening within the lattice structure and thus increasing the dislocation energy and decreases the solubility of the alloying elements, chromium, zirconium and titanium in the case of the invention.

The copper based precipitation hardenable alloy of the invention is advantageously used because of the improvements in properties such as electrical conductivity and mechanical strength in many solutions in electric, electronic and welding industries.

The invention is described in more details referring to the appended drawings wherein

FIG. 1 illustrates an additive ternary copper-chromium-phosphorus (CuCrP) phase diagram close to the copper corner (100% copper) at the temperature of 600° C., and

FIG. 2 shows the test results for the value of stress in percents (%) remaining after the 100 hour-test at the temperature of 175° C. for a copper chromium zirconium phosphorus (CuCrZrP) alloy.

Phosphorus addition of up to 500 ppm to a chromium, zirconium and titanium bearing copper based precipitation hardenable alloy has a direct impact on the electrical conductivity. The phosphorus addition reduces the solubility of the alloying element, chromium, zirconium or titanium in the terminal crystal structure of face centered cubic copper (fcc-Cu). For instance chromium forms thermally stable phosphides, such as Cr₃P and CrP₄, but no double phosphide with copper. One reason is, that chromium has the terminal crystal structure of base centered cubic (bcc), instead of face centered cubic (fcc) for copper.

The solubility of chromium into copper at the presence of phosphorus is illustrated in FIG. 1 referred from Villars P., Prince A., Okamoto H., Handbook of Ternary Alloy Phase Diagrams, Vol 7 & 8, ASM International, Metals Park (Ohio), 1998. FIG. 1 shows an additive ternary Cu—Cr—P in the copper corner (100% copper) at the temperature of 600° C. The term “additive” means that no ternary interactions are taken into account, which approximation would not bring large differences in the fcc-solid solution due to the very small solubilities, in particular in the case of chromium in copper. As the solubility of chromium in solid copper is less than 0.01% by weight the effect on the solubility of phosphorus is very small. On the other hand, the solubility of phosphorus in copper chromium alloys is limited by the Cr₂P phosphide to a fraction of the solubility of phosphorus in the copper phosphorus binary alloy.

Additional isotherms at higher temperatures have been evaluated and indicate that the chromium phosphide (Cr₂P) extending from the chromium phosphorus edge of the ternary system limits the solubility of phosphorus in typical copper chromium compositions with about 0.1% by weight chromium. The maximum solubility of phosphorus in binary fcc-Cu and bcc-Cr alloys at the temperature of 600° C. is about 100 ppm. Beyond that concentration, CrP phosphide precipitates from two-phase system of the fcc-Cu and bcc-Cr. It is also recognized from FIG. 1 that phosphorus additions at high concentrations systematically lower the solubility of chromium in the fcc-Cu alloy.

Zirconium forms a ternary compound and is stable with a stoichiometry of copper-zirconium phosphide of Cu₂ZrP. Further, a binary compound of zirconium phosphide (Zr₅P₄) precipitates from the super saturated zirconium-phosphorus-copper alloy during casting or aging. This binary compound has no effect on conductivity and in effect reduces the solubility of zirconium within copper.

Titanium forms with phosphorus for instance phosphides of Ti₃P and TiP. Titanium also forms a ternary compound with copper and phosphorus (Cu₂TiP), which is stable. Also in the case of having titanium as an alloying element in the precipitation hardenable copper based alloy the formation of the binary and ternary compounds help to increase the electrical conductivity and tensile strength of copper.

The influence of phosphorus to the copper chromium, copper zirconium and copper titanium system increases the yield strength, tensile strength and hardness with no effect on ductility. Another advantage of the formation of phosphides is the effect on the recrystallization temperature. The influence of the phosphides enables strain hardened or cold worked material to be exposed to the temperature range of 800-1200° C. where most other high conductivity alloys would lose most of their properties achieved by strain hardening.

To determine the effects of phosphides on strain harden material at elevated temperatures a stress relaxation test was performed on the alloy (CuCrZrP) containing copper, 0.75% by weigth chromium, 0.06% by weight zirconium and varying contents of phosphorus. The value of stress in percents (%) remaining after the 100 hour-test at the temperature of 175° C. is shown in FIG. 2. As the FIG. 2 shows the amount of stress remaining in the CuCrZrP alloy with the higher phosphorus content is almost 100%. However, it has to be pointed out that the alloy processing is critical in achieving the superior properties. Based on FIG. 2, phosphorus contents above 550 ppm have been found to have a negative impact on the electrical conductivity and mechanical properties of the chromium, zirconium or titanium containing copper based precipitation hardenable alloys alloyed by phosphorus. 

1. A copper based precipitation hardenable alloy containing at least one of the elements chromium, zirconium or titanium, wherein the copper based precipitation hardenable alloy is alloyed by phosphorus.
 2. The copper based precipitation hardenable alloy according to claim 1, wherein the alloy contains 100 to 500 ppm phosphorus.
 3. The copper based precipitation hardenable alloy according to claim 1, wherein the copper based precipitation hardenable alloy is a copper chromium alloy containing at least 98.5% by weight copper.
 4. The copper based precipitation hardenable alloy according to claim 3, wherein the alloy contains 0.1 to 1.5% by weight chromium.
 5. The copper based precipitation hardenable alloy according to claim 1, wherein the copper based precipitation hardenable alloy is a copper zirconium alloy containing at least 98.5% by weight copper.
 6. The copper based precipitation hardenable alloy according to claim 5, wherein the alloy contains 0.01 to 0.25% by weight zirconium.
 7. The copper based precipitation hardenable alloy according to claim 1, wherein the copper based precipitation hardenable alloy is a copper chromium zirconium alloy containing at least 98.5% by weight copper.
 8. The copper based precipitation hardenable alloy according to claim 7, wherein the alloy contains 0.1 to 1.5% by weight chromium and 0.01 to 0.25% by weight zirconium.
 9. The copper based precipitation hardenable alloy according to claim 1, wherein the copper based precipitation hardenable alloy is a copper titanium alloy containing at least 96.5% by weight copper.
 10. The copper based precipitation hardenable alloy according to claim 9, wherein the alloy contains 0.05 to 3.4% by weight titanium.
 11. The copper based precipitation hardenable alloy according to claim 2, wherein the copper based precipitation hardenable alloy is a copper chromium alloy containing at least 98.5 % by weight copper.
 12. The copper based precipitation hardenable alloy according to claim 2, wherein the copper based precipitation hardenable alloy is a copper zirconium alloy containing at least 98.5% by weight copper.
 13. The copper based precipitation hardenable alloy according to claim 2, wherein the copper based precipitation hardenable alloy is a copper chromium zirconium alloy containing at least 98.5% by weight copper.
 14. The copper based precipitation hardenable alloy according to claim 2, wherein the copper based precipitation hardenable alloy is a copper titanium alloy containing at least 96.5% by weight copper. 